Implantable medical electrical stimulation and/or sensing leads are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation. In the field of cardiac stimulation and monitoring, endocardial leads are placed through a transvenous route to locate one or more sensing and/or stimulation electrodes along, or at the distal end of, the lead in a desired location within a heart chamber or interconnecting vasculature. In order to achieve reliable sensing of the cardiac electrogram and/or to apply stimulation that effectively paces or cardioverts the heart chamber, it is necessary to accurately position the electrode surface against the endocardium or within the myocardium at the desired site and fix it during an acute post-operative phase until fibrous tissue growth occurs.
The pacemaker or defibrillator implantable pulse generator (IPG) or the monitor is typically coupled to the heart through one or more of such endocardial leads. The proximal end of such leads typically is formed with a connector that connects to a terminal of the IPG or monitor. The lead body typically comprises one or more insulated conductive wires surrounded by an insulating outer sleeve. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. An endocardial cardiac lead having a single stimulation and/or sensing electrode at the lead distal end and a single conductive wire is referred to as a unipolar lead. An endocardial cardiac lead having two or more stimulation and/or sensing electrodes at the lead distal end and two or more conductive wires is referred to as a bipolar lead or a multi-polar lead, respectively.
In order to implant an endocardial lead within a heart chamber, a transvenous approach is utilized wherein the lead is inserted into, and passed through, the subclavian, jugular, or cephalic vein and through the superior vena cava into the right atrium or ventricle. An active or passive fixation mechanism is incorporated into the distal end of the endocardial lead and deployed to maintain the distal end electrode in contact with the endocardium position.
More recently, endocardial pacing and cardioversion/defibrillation leads have been developed that are adapted to be advanced into the coronary sinus and coronary veins branching therefrom in order to locate the distal electrode(s) adjacent to the left ventricle or the left atrium. The distal end of such coronary sinus leads is advanced through the superior vena cava, the right atrium, the ostium of the coronary sinus, the coronary sinus, and into a coronary vein communicating with the coronary sinus, such as the posterior lateral vein, mid-cardiac vein, or the great cardiac vein. Typically, coronary sinus leads do not employ active fixation mechanisms and instead rely on the close confinement within these vessels, and general lead body properties of stiffness and shape, to maintain each electrode at a desired site.
Routing an endocardial lead along a desired path to implant the electrode or electrodes in a desired implantation site, either in a chamber of the heart or in the selected cardiac vein or coronary artery, can be difficult. This is particularly true for navigating leads through the coronary sinus and into a branching vein on the left myocardium. Anomalies in the vascular anatomy and the number of branch veins associated with the anatomy make locating the desired path challenging.
Several common approaches have been developed to place electrodes within the left side of the heart. According to one approach, a guide catheter is steered into the desired location in the vasculature. A lead is then fed through the inner lumen of the catheter such that the lead electrode(s) are positioned at predetermined locations. The guide catheter may then be withdrawn. This type of approach is described in commonly assigned U.S. Pat. Nos. 6,006,137, 5,246,014, and 5,851,226 incorporated herein by reference. The described systems employ highly flexible, catheters surrounding the lead body. One difficulty with systems that completely surround the cardiac lead is that permanently implantable endocardial leads are formed typically with a proximal connector end assembly having a diameter exceeding that of the lead body. These connectors are designed to conform with an industry standard so that the connector mates with an IPG standard connector bore. Consequently, the introducer has to be made large enough to fit over the enlarged diameter connector end assembly. This detracts from the ability to advance the introducer and lead assembly through small diameter blood vessels. A smaller introducer that is designed to be split or slit may be used in the alternative, but these types of introducer are more difficult to manufacture. Yet another approach involves use of a lead without a connector end assembly, or with a smaller, non-conforming connector end assembly. Such a lead must be coupled to an adapter before it conforms to a standard, which is both inconvenient, and can result in a diminished reliability.
Another approach to lead placement involves the use of a guidewire that is steered into a desired location within the vasculature. The lead body is then tracked over-the-wire and the wire is withdrawn. According to this design, the guidewire passes through an inner lumen of the lead for an entire length of the lead. This results in a significant amount of friction that can make lead placement difficult. Additionally, since the lead must include an inner lumen for the guidewire, the size of the lead is at least somewhat dictated by the size of the guidewire. Moreover, to accomplish lead placement in this manner, the lead must be stiff enough to allow it to be advanced over the guidewire through the tortuous curves of the vasculature.
One way to minimize drag is to provide a “siderail” lead that includes means for tracking a guidewire at only a predetermined portion of the lead distal tip. This type of lead system is disclosed in U.S. Pat. No. 5,003,990, also incorporated by reference herein. This system relies on a guidewire and a carriage that releasably engages the distal electrode and is pushed along the guidewire as the lead body is pushed along the transvenous path. The guidewire is first introduced along one of the above-described desired paths, and the carriage engaging the distal electrode is placed over the proximal end of the guidewire and introduced into the blood vessel. Force is exerted against the lead body to push the carriage and the distal end of the lead body distally along the guidewire until the distal electrode is near to the desired site. The electrode is disengaged from the carriage, and the carriage is retracted along the guidewire by pulling on another wire attached to the carriage or by the retraction of the guidewire. Such retraction of the relatively bulky carriage presents the possibility of damage to an artery or vein by the carriage. Because of unintended movement of the guidewire that typically occurs during the process of disengaging the electrode from the carriage, the distal end of the lead will not necessarily be positioned at the desired implant location. As a result, some other mechanism may be needed to re-position the electrode. This adds time and complexity to the implant procedure.
In a further approach disclosed in U.S. Pat. No. 5,304,218, incorporated by reference herein, a cardiac lead is formed with a channel in the distal tip that receives a guidewire that has already been advanced through the path to the cardiac implantation site. The lead is pushed over the guidewire to the cardiac implantation site where the guidewire is withdrawn and the lead is either fixed in place or left at the cardiac implantation site. There is no disclosure of how this approach could be used to advance a cardiac lead having an active or passive fixation mechanism at or near the channel in the distal end of the lead body.
In both of the above-described approaches, the lead body must possess sufficient column strength to allow it (as well as the carriage of the '990 patent) to be pushed from the proximal end outside the patient's body and along the guidewire. The lead body diameter and/or construction materials that are required in order to make the lead body stiff enough to accomplish this over-the-wire advancement method necessarily make the lead body larger and less flexible than is desirable to withstand the rigors of chronic flexing as described above. The over-the-wire approach is classically employed in advancement of balloon catheters for Percutaneous Transluminal Coronary Angioplasty (PTCA) use which is intended to be of short duration.
Other similar over-the-wire approaches have also been disclosed. U.S. Pat. No. 6,129,749 to Bartig et al., which is incorporated herein by reference, describes a lead body having an electrode support structure at the distal tip that includes a lumen for a guidewire. The support structure is passed over the guidewire until the electrode is positioned in the desired location, and the guidewire is then removed leaving the electrode in place. U.S. Pat. No. 5,755,765 to Hyde et al., incorporated herein by reference, discloses a lead having a guide loop near the distal tip for advancing over a guidewire to an implant site. U.S. Pat. No. 5,902,331 to Bonner et al., which is incorporated herein by reference, describes a tracking mechanism that may be coupled to a lead body, and that may be pushed via a pusher over an elongated guide body to a desired implant site. U.S. Pat. No. 5,803,928 to Tockman et al., which is incorporated herein by reference, discusses an over-the-wire pacing lead having a side access port for being slid over a guidewire to a desired implant position.
One problem with the systems described in the foregoing patents is that no anchoring mechanism is provided to maintain the guiding device such as the guidewire in a stationary position while the leads are being advanced to the desired implant site. This can cause the guidewire to become dislodged. What is needed is some type of anchoring mechanism that can be utilized while one or more leads are steered into position. This anchoring mechanism must be retractable so that the guiding device may be withdrawn from the vasculature without disrupting lead placement. Ideally, the guiding device could also be used to facilitate fixation of the electrode to the vasculature.